The manufacture of corrugated board typically involves the following steps:    (a) fluting a first cellulosic sheet by passing it between heated corrugating rolls so that the obtained “corrugating medium” has a substantially sinusoidal or serpentine cross-section;    (b) applying adhesive to the protruding flute tips on at least one side of the corrugating medium; and    (c) bonding a non-corrugated or planar cellulosic sheet (“liner sheet”) to the adhesive-coated flute tips.
The resulting product, having corrugating medium on one side and a liner sheet on the other, is called single-faced corrugated board. It can be used, for example, as a liner or buffer material within a container. More commonly, however, adhesive is applied to the flute tips on both sides of the corrugating medium and a second liner sheet is applied, effectively sandwiching the corrugating medium between the two liner sheets. The resulting product is known as double-faced corrugated board and is commonly used for the manufacture of cardboard boxes and other such containers or packaging materials. For increased rigidity or strength, several such single-faced and/or double faced boards can be combined to produce multiple-wall corrugated board.
To ensure proper adhesion, the step of bonding the corrugating medium to its one or more liner sheets is normally carried out under pressure and at temperatures of about 150 to 200° C. These high temperatures encourage curing of the adhesive and evaporation of any excess water.
Adhesives used in the production of corrugated board are selected on the basis of several factors including, for example, cost and the intended use of the finished corrugated product. Starch-based adhesives, such as Stein Hall adhesives, are the most commonly used because of their desirable bonding properties, ease of preparation and low cost.
Nonetheless, there is a continued drive in the industry to reduce the cost of producing corrugated board. A number of areas have been targeted in this respect including, for example, energy efficiency. One of the main costs incurred in the production of corrugated board is, indeed, the energy required for heating. Heating is used both to gelatinise the starch-based adhesives (thereby increasing viscosity and bonding between sheets) and for drying (i.e. to remove any excess water remaining on the board from the liquid adhesives). In order to reduce heating requirements, one approach has been to reduce the water content of the corrugating adhesives. Put another way, this means increasing their dry solid content. This concept can, however, only be taken so far as adhesive viscosity (directly linked to dry solid content) must be strictly controlled. A too viscous adhesive would be difficult to apply to the flute tips and could cause clogging and flow problems on the corrugating machine. What's more, high viscosity can cause excess transfer of adhesive to the paper (add-on) thereby dramatically increasing adhesive costs.
Another approach has been to reduce the overall adhesive add-on. Again, this naturally reduces the quantity of water that will need to be evaporated off the formed board. It will also cut raw material costs (e.g. the quantity of adhesive needed per square meter of board produced). Unfortunately, add-on cannot be reduced below a certain level without having a detrimental effect on bond strength and, therefore, on board quality.
Moving away from the adhesives themselves, another area that has been targeted in an attempt to cut costs has been the quality (thickness, weight) of the cellulosic sheets used to form the corrugated board. As paper pulp becomes more expensive due to increased demand, there is a need to decrease the use of paper to make corrugated containers. A common process is to add materials such as starch to strengthen papers used in corrugated containers. This strengthening is known as “sizing”. Sizing is important to increase the ability of corrugated containers to resist collapsing when they are stacked on each other while filled with goods. This important property is known in the industry as “stacking strength”. Corrugated board manufacturers use several methods to measure box stacking strength, including the Edge Crush Test (ECT—TAPPI test no. T 838 om-07—used to measure the strength of the corrugated board itself) and the Box Compression Test (BCT—TAPPI test no. T 804 om-06—used to measure the crushing strength of a standard box made with corrugated board). In order to improve ECT and/or BCT performance it is generally accepted that higher strength papers are needed. Two well-known tests to indicate paper strength are the Short Span Compression Test (SCT—TAPPI test no. T826 (wd-01)) and the Ring Crush Test (RCT—TAPPI test no. T818 (cm97)).
In order to meet multiple customer needs, the corrugated board manufacturer needs to buy papers with certain strengths and must inventory multiple papers. If the strength of the paper could be adjusted by the corrugator, this would provide a more flexible process wherein the strength of the corrugated box could be adjusted using fewer papers. Obviously, though, any reduction in sheet quality will also have a negative impact of board strength and functionality. As such, the possibilities of savings in this area are limited.
There is therefore still a clear need to develop new and improved corrugating adhesives and better, more economic processes for producing corrugated board which do not have a detrimental effect on the quality of the final product. The present invention addresses these needs.